For literature dealing with the background of this invention, reference may be had to the patents and publications described in or cited in the files of the above-identified applications.
Generally speaking, for environmental protection purposes it is desirable to remove contaminants from the flue gases of fossil fuel power plants and, particularly, to remove the sulfur oxides which are generally present in the flue gases produced by most fossil fuels.
While various methods have been proposed for this purpose, a most effective technique is to subject the flue gases to a wet scrubbing with a scrubbing liquid which contains calcium compounds e.g. calcium carbonate, calcium hydroxide or calcium oxide, these compounds being referred to generally as "lime" since all serve essentially the same purpose, namely, that of providing the calcium ion in the aqueous scrubbing liquid so that this calcium ion can combine with anions formed by the sulfur oxides to produce, for example, calcium sulfite and calcium bisulfite by absorption of sulfur dioxide from the flue gas. When reference is made to calcium sulfite, therefore, it should be understood that calcium bisulfite may be included.
A scrubbing tower for this purpose is generally upright and is provided with a flue gas inlet at a lower portion of the tower, connected to the power-plant boiler flue, a flue gas outlet in the region of the head of the tower or column, means between the inlet and the outlet for spraying a scrubbing liquid into the gas, and a sump for collecting the scrubbing liquid below this inlet.
The sump may be provided with means for enabling calcium sulfite and/or calcium sulfite which can form in suspension in the scrubbing liquid to be withdrawn, while the flue gas outlet at the head of the tower can be provided with a radial outlet for the flue gas and a droplet separation for removing droplets of scrubbing liquid entrained with the gas.
Conventional droplet separators for this purpose utilize cyclone principles for separating the entrained liquid from the gas and are characterized by the need for high flow velocities so as to operate at efficient ranges for such separators. The high flow velocities, in turn, mean that the pressure drop is considerable and the resulting energy cost is high.
Accordingly, droplet separators have been provided at the head of the tower or column which utilize baffle or impingement separation principles. These separators can be provided in sectors around a central space, an annular collecting passage being formed outwardly of the array of sectors, each of which may be an impingement baffle separator formed from a number of plates.
The flue gas rising centrally into the separator at the head of the column then passes outwardly through the baffle sectors and is collected in the outflow passage to flow to the common radial outlet. The sectoral baffle units are also described herein and in the prior applications where applicable as droplet separator registers.
Naturally, the droplet separator is designed and dimensioned, i.e. the number of sectors or registers is selected and the number of plates and their arrangement are designed to accommodate the maximum flow rate which can be expected.
However, power-generating boilers are frequently operated at less than full output with a corresponding reduction in flue gas generation and, as a result, the scrubber may be operated at less than full gas scrubbing capacity. Under these circumstances the efficiency of the baffle separators drops sharply.